In the photographic industry, the need to provide photographic film and paper with antistatic protection has long been recognized. In instances where the main need for antistatic protection is prior to processing, materials that provide temporary antistatic protection are acceptable. By temporary, we mean that the antistatic material may react or dissolve in the photographic processing solutions and lose some or all of its ability to provide antistatic protection. Permanent antistatic materials, on the other hand, are those that do not lose their antistatic property even after the photographic product has been processed. The latter kind of antistatic material is more desired in products which are subjected to high speed transport after processing, such as microfilm materials, or which have a magnetic recording layer incorporated in the coating, in which case static discharge can contribute to magnetic signal noise. From a product performance standpoint, a permanent antistat material would minimize the amount of dust adhering to a product. Such permanent antistatic properties can be obtained by ionic conductors like ionic conductive polymers as well as by electronic conductors such as fine particles of crystalline metal oxides. The antistatic properties of ionic conductors are typically sensitive to humidity conditions, whereas electronic conductors are not. Therefore, the fine particles of crystalline metal oxides are the preferred antistatic materials.
There are several types of crystalline metal oxide particulates which are used to prepare optically transparent antistatic coatings. Examples of these are disclosed in U.S. Pat. Nos. 4,275,103, 4,394,441, 4,416,963, 4,418,141, 4,431,764, 4,495,276, 4,571,361, 4,999,276 and 5,368,995. Preferred materials are antimony doped tin oxide, aluminium doped zinc oxide, and metal antimonates. For coatings used in imaging applications, the particle size of these powders should be small, in order to minimize light scattering and haze. The high propensity for scattering light, by these particles is due to their relatively high refractive index. In order for thin coatings of these materials to possess the required antistatic property, the volume fraction of these materials in the coatings needs to be relatively high (&gt;50% by volume). The need for high volume fraction of the metal oxide particles is that they should form a gelled network of particles which typically occurs beyond a characteristic percolation threshold in the volume fraction. Above the threshold value of the volume fraction, the surface conductivity is relatively high. Below the threshold value, the surface conductivity drops several fold and reaches a low value. The surface conductivity of coatings of these materials typically has a value desired for photographic applications provided the volume fraction of the antistatic material is 50 volume percent or higher. In order to form a coating the antistatic materials are coated with a polymeric binder. Binder materials in coatings are either water soluble or water dispersable polymers, such as gelatin or film forming latexes, or solvent soluble polymers such as polyurethanes. Coatings of these polymers have mechanical properties of glassy materials, once the coatings have been dried. However, the integrity and mechanical strength of coatings of composite materials, in which the binder is less than 50 percent by volume, is low, especially when the particulate filler material such as crystalline metal oxide particles is very different in mechanical properties from the binder material which is glassy. Thus, the requirements of the coating, to have a high electrical conductivity and to have reasonable mechanical strength, is difficult to acheive simultaneously.
U.S. Pat. No. 5,340,676 discloses the use nonswellable, insoluble polymer latex particles along with gelatin, in order to increase the conductivity of the coating at relatively lower volume fractions of the conductive metal oxide particles. However, while the conductivity is improved by addition of non-swellable polymer latex particles, the adhesion of other layers of the imaging element to the conductive layer is compromised with increasing latex content of the conductive layer. U.S. Ser. No. 08/816,650, now U.S. Pat. No. 5,849,472, discloses the use of carboxylic acid containing polymer latex particles for similar reasons, with the added advantage of increasing the permeability of the coated layer to processing solutions required to produce images. U.S. Pat. No. 5,466,567 describes the addition of pre-crosslinked gelatin particles to the soluble gelatin binder of the conductive layer to improve conductivity. However such particles are difficult to prepare and disperse in water and their size and size distribution are not easily controlled.
Antistatic coatings, with adequate surface conductivity, but with a low volume fraction of metal oxide can also be obtained by using fibrous powders of these materials, as disclosed in U.S. Pat. No. 4,845,369. The aspect ratio of these fibers is typically greater than 50. However, due to their larger size in one dimension and high refractive index, they have greater potential to scatter light than the small particles. Secondly, the fibrous materials are more difficult to manufacture and, thus, cost more.
Thus, it is desirable to produce coatings of antistatic materials, having small conductive particles whose volume fraction in the coating is less than 50 percent, which have good electrical conductivity (resistivity &lt;10.sup.10 ohm/square) and which have good adhesion to other layers of the imaging element coated over them such as curl control layers or light sensitive layers.